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Special Notice #219: Eclipse of the symbiotic star CI Cygni

October 12, 2010: As noted by U. Munari et al (ATel #2913, 2010 October 9), the symbiotic star CI Cygni is currently undergoing an eclipse of the outbursting star. CI Cyg has declined from about magnitude 9.7 in late August 2010 to V=11.1 currently. Observations of this star -- including visual estimates, spectroscopy, CCD time-series, and calibrated photometry -- are encouraged. The star is going into eclipse as predicted, as CI Cyg has a known eclipse period of 833 days. However, the eclipse is happening as the object is in outburst (see AAVSO Special Notice #215, 2010 August 24), and the decline is rapid with a large amplitude. As Munari noted (ATel #2913), the eclipse offers researchers the oppportunity to study the extended environment of this system in outburst without being overwhelmed by light from the central component.

We note that calibrated photometry can provide more complete information about the physical properties of the system, and observers are encouraged to fully calibrate and transform their multi-filter observations. Single-filter time-series observations are acceptable; unfiltered observations are discouraged due to the red color of the system.